A FIELD COMPARISON OF INDICATORS OF SUBLETHAL STRESS IN THE SALT-MARSH GRASS SPARTINA PATENS

There is a need for research into bioindicators of stress in threatene d plant communities such as coastal wetlands. Land subsidence, diversi on of sediment, and salt water intrusion produce stresses associated w ith waterlogging, elevated salinity, and nutrient depletion, Temporal and spatial environmental variation (sell redox potential, interstitia l water salinity, pH, ammonium and phosphorus, and cation and trace me tal concentrations) was analyzed near Lake de Cade, Louisiana, in a br ackish marsh which is a mosaic of healthy plant communities interspers ed with areas where wetland loss is occurring. Environmental variation was related to indicators of stress in Spartina patens, which include d variables derived from the adenine nucleotide levels in plants, leaf spectral reflectance, leaf proline concentrations, and shoot elongati on. In a comparison of burned and unburned sites, streamside and inlan d marsh, and along a salinity gradient, among-site differences were fo und in spectral reflectance and adenine-nucleotide-related indicators. Although it was difficult to relate a single causal environmental var iable to the response of a specific indicator, spectral reflectance in the visible light range responded to salinity or to elements borne in seawater; and adenine-nucleotide indices were sensitive to nutrient a vailability, The ability of indicators to detect plant responses chang ed during the growing season, suggesting that they were responding to the changing importance of different environmental factors, In additio n, some reflectance indicator responses occurred along salinity gradie nts when salinity differences were less than those that were found to have ecologically meaningful effects in greenhouse experiments. A mult ivariate numerical approach was used to relate environmental variation with indicator responses. We concluded that factors which in combinat ion cause the degradation and loss of Louisiana wetlands produce envir onmental conditions that are only subtly different from those in vigor ously growing marsh communities.